1. Field of the Invention
This invention relates generally to integrated circuits, and more particularly to MOSFET structures for integrated circuits.
2. Description of the Prior Art
A common type of integrated circuit device is the metal-oxide field-effect transistor (MOSFET). A MOSFET includes a source region, a drain region, a channel region extending between the source and drain regions, and a gate structure provided over the channel region. The gate structure includes a conductive gate, and a thin oxide layer separating the gate from the channel region.
Operationally, a biasing potential is provided between the source and drain regions of a MOSFET. The standard MOSFET source/drain bias voltage for most integrated circuits is five volts. When a control voltage is applied to the gate, a depletion zone can be formed within the channel region, permitting current to flow from the source region to the drain region. Thus, the MOSFET can serve as a switch responsive to the level of the control voltage applied to the gate.
A troublesome phenomenon known as the "hot electron effect" occurs as a MOSFET is made smaller. When a MOSFET is reduced in size, the distance between the source region and the drain region becomes smaller and, assuming a constant source/drain bias voltage, the electric field strength near the drain region increases dramatically. The high electric field strength accelerates the electrons rapidly, resulting in the formation of "hot" electrons. The hot electrons tend to become trapped in the thin oxide layer of the gate structure in the vicinity of the drain region. As charge accumulates in the thin oxide, the threshold voltage of the MOSFET changes, and the speed and efficiency of the MOSFET can be greatly reduced.
In FIG. 1, a prior art MOSFET structure which reduces the hot electron effect includes lightly doped tips at the source and drain regions. Since the drain tip is more lightly doped than the drain region itself, the electric field near the channel is softened. In consequence, the channel current will not produce as many hot electrons, reducing the hot electron effect. This prior art structure is known as a "lightly doped drain" or "LDD" structure.
While the prior art structure of FIG. 1 reduces the hot electron effect, it does not completely eliminate it. Another prior art structure known as the "buried lightly doped drain" or "BLDD" structure, which is a slight improvement over the LDD structure, is shown in FIG. 2. Like the LDD structure, the BLDD structure includes a pair of lightly doped tips near the source region and drain region of the transistor. However, the BLDD additionally includes a pair of buried implant regions beneath the tip regions of the transistor which are more lightly doped than the drain and source regions, but which are more heavily doped than the tip regions. The drain buried implant region causes the channel current to deflect downwardly because it has a lower resistance than the drain tip region.
Theoretically, the downward deflection of the channel current with BLDD structure reduces the number of hot electrons collecting in the thin oxide. However, since the buried implant region is more heavily doped than the lightly doped regions, the electric field strength of the BLDD structure is larger than the electric field generated with the structure of FIG. 1, causing a greater acceleration of the electrons of the channel current. As a result, the immunity of the BLDD structure to the hot electron effect is only slightly better than the immunity of the LDD structure.
A problem with the BLDD structure is the potential for "punch-through" where current flows between the buried implant regions, as indicated in broken lines in FIG. 2. If this happens, the MOSFET will have excessive leakage current and will not be fully responsive to the control voltage applied to the gate.
The LDD and BLDD structures can be manufactured by similar processes. For example, a polysilicon gate can be first formed over the thin oxide layer. Then, the lightly doped tip regions can be formed by ion implantation. The tip regions protrude into the channel region due primarily to a "side scattering" effect. For the BLDD structure, the buried implant regions can next be formed by high-energy ion implantation. The buried implant regions extend even further into the channel region because the high energy required for the buried implant regions increases the side scattering effect. Next, for both the LDD and BLDD devices, oxide spacers are formed along the sides of the polysilicon gate, and the source and drain regions are formed by ion implantation. Again, the source and drain regions protrude toward the channel region due to the side scattering effect.